Fluid coking with alumina seeds



Nov. 29, 1955 R. P. CAHN ETAL 2,725,349

FLUID COKING WITH ALUMINA SEEDS Filed June 2, I954 ROBERT P. CAHN HENRY ERNST JR INVENTORS ATTORNEY United States Patent FLUID COKING WITH ALUMIYA SEEDS Robert P. Cahn, Elizabeth, and Henry Ernst, Jr., Fanwood, N. 3., assignors to Essa Research and Engineering Company, a corporation of Delaware This invention relates to improvements in the coking of heavy hydrocarbon oils where a coking charge stock is contacted at coking temperatures with coke particles maintained in the form of a dense turbulent fluidized bed. More particularly, it relates to an integrated process of this nature wherein fine alumina particles are utilized as growth seeds for the process. It also relates to carbon electrodes made from fluid coke particles containing these alumina seeds. I

There has recently been developd an improvd process known as the fluid coking process for the production of lower boiling distillates from heavier fractions. The fluid coking unit consists basically of a reaction vessel or coker and a heater or burner vessel. In a typical operation the heavy oil to be processed is injected into the reaction vessel containing a dense turbulent fluidized bed of hot inert solid particles, preferably coke particles. Uniform temperature exists in the coking bed. Uniform mixing in the bed results in virtually isothermal conditions and effects instantaneous distribution of the feed stock. In the reaction zone the feed stock is partially vaporized and partially cracked. Product vapors are removed from the coking vessel and sent to a fractionator for the recovery of gas and light distillates therefrom. Any heavy bottoms is usually returned to the coking vessel. The coke produced in the process remains in the bed coated on the solid particles.

The heat for carrying out the endothermic coking reaction is generated in the burner vessel. A stream of coke is transferred from the reactor to the burner vessel employing a standpipe and riser system; air being supplied to the riser for conveying the solids to the burner. Sufi'icient coke or carbonaceous matter is burned in the burning vessel to bring the solids therein up to a temperature sufficient to maintain the system in heat balance. The burner solids are maintained at a higher temperature than the solids in the reactor. About 5% of coke, based on the feed, is burned for this purpose. This amounts to approximately 15% to 30% of the coke made in the process. The unburned portion of the coke represents the net coke formed in the process. This coke is preferably withdrawn from the burner, normally cooled and 1 sent to storage.

Heavy hydrocarbon oil feeds suitable for the coking process are heavy or reduced crudes, vacuum bottoms, pitch, asphalt, other heavy hydrocarbon residua or mixtures thereof. Typically, such feeds can have an initial boiling point of about 700 R, an A. P. I. gravity of about 0 to 2 0, e. g., 19, and a Conradson carbon residue content of about 5 to 40 weight per cent. (As to Conradson carbon residue see ASTM Test D-180-52.)

It is preferred to operate with solids having an average particle size ranging between 100 and 1000 microns in diameter with a preferred particle size range between 150 and 400 microns. Preferably not more than 5% has a particle size below about 75 microns, since small particles tend to agglomerate or are swept out of the system with the gases.

ICC

A more complete description of this technique of fluid solids coking can be obtained by reference to copending application entitled, Fluid Coking of Heavy Hydrocarbons and Apparatus Therefor, Serial No. 375,088, filed August 19, 1953, by Pfeilfer et al. The method of fluid solids circulation described above is well known in the prior art. Solids handling technique is described broadly in Packie Patent No. 2,589,124, issued March 11,1952.

Because only a part of the coke deposited is burned to supply heat, the coke particles grow in size during the process and must be withdrawn as product. In order, to counteract ehe growth process and to maintain a constant particle size coke inventory, fine coke particles must be added continuously to serve as growth seeds. The seeds required can be supplied either by grinding withdrawn coke in external equipment or by devices in the unit itself. The number of seed particles required is approximately equal to the number of coke particles withdrawn as product. However, because the seed coke is much smaller in diameter than the average of the withdrawn coke, the weight of seed required is only a small percentage of the weight of the coke withdrawn. Seed coke particles typically have a size distribution predominantly of to 150 microns in diameter.

Equipment, including grinding equipment, for making seed coke can increase the cost of a fluid coking unit by 10% or more. It is therefore desirable to be able to seed the process economically.

This invention provides an improved integrated fluid coking process wherein fine alumina particles are utilized as the seed material. The alumina is consequently occluded in .the coke particles-formed. This occluded material actually improves the quality of the coke when it is used in carbon anodes for the electrolytic aluminum preparation process. The occluded alumina dissolves in the bath and is collected at the cathode as aluminum.

The alumina that can be used include all the well known varieties of natural and synthetic alumina including the amorphous and crystalline forms. A preferred seed material is the pure alumina, such as produced by the caustic soda extraction of bauxite known as the Bayer Process.

In this process, the bauxite is finely ground and charged into digesters along with spent caustic liquor from a previous cycle and sufficient lime and soda ash to give the necessary concentration of sodium hydroxide. The bauxite, lime and soda ash are weighed continuously and transferred to the digesters by belt conveyors. The digesters operate under elevated (less than 100 p. s. i. g.) steam pressure for sufiicient, time to bring some to of the alumina into solution with the caustic soda. The sodium aluminate solution is then separated from the insoluble residue by settling in huge tanks and filtration using a large battery of Kelly filters. The residue, which contains a considerable percentage of iron oxide, is known as red mud and is dumped in an area outside the boundary of the plant proper. It contains as much as 20% of the original alumina (recovery of more begins to bring impurities into solution) and can be reworked if circumstances warrant it.

- The sodium aluminate solution is reduced to atmospheric pressure and passed to a number of large, air agitated, open top precipitation tanks where it is seeded with alumina trihydrate from a previous cycle and slowly cooled. Alumina trihydrate is formed by hydrolysis of sodium aluminate in the presence of the seed material. After precipitation has been completed, the granular trihydrate is removed from the caustic solution remaining by thickeners and filtration using rotary filters. The trihydrate is then calcined at about 1800 F. to remove free water and reduce combined water to a maximum of 0.5%. Large rotary, oil-fired kilns are employed. The

Patented Nov. 29, 1955 alumina need not be calcined as this is done in the coker and coke calciner.

The fine alumina particles utilized have a diameter size distribution predominantly of 75 to 150 microns in diameter. If uncalcined alumina '(from Baiye'r Rrocess) is used, theu'pper particle size is not too critical, as the 'p'articles disintegrate on dehydration-at cok'er temperatures.

As explained previously'the number of -s'eed'p'arti'cles required is approximately equal in-numb'er' to the larger coke particles withdrawn as product. Becauseof the smaller size of the alumina and density considerations, the weight of the alumina'seed required is only 'a small percentage of the weight of the coke withdrawn.- "For example, the theoretical seed requirement for 100 microns (average) seed and 300 micron (average) ".coke withdrawal is of the net c'oke make. In'p'ra'ctice some variation from theoretical :is'required to allow for seed loss, "and attrition of seed and coke. A range of seed requirement of 3-10 'wtfper cent on net coke .make is therefore required for control of'the process. Approxima'tely the same 'co'ncentrationof alumina is found in uncalcined coke product. quired seed addition rate can be utilized to give a coke of higher aluminum contentof'desired.

The alumina utilized because of physical characteristics cannot be used as abrasives.

This invention will be better'understood'by reference toan example and the flow diagram shownin the drawmg.

In'the drawing the numeral '1 is-acoking vessel constructed of suitable materials for operation at 950 .F. A bed of coke particles preheated to a sufficient temperature, e. 'g., 1125 F. to establish the required bed temperature of 950 F. is made up of suitable particles of 150-400 microns. The bed of solid particles reaches an upper level indicated by the numeral 5. The bed 'is fluidized by means of a gas such as steam entering the vessel at the stripping portion near the bottom thereof via pipe 3. The fluidizing gas plus vaporsfrom the cold ing reaction pass upwardly through the vessel at avelocity'of l ft./sec. establishing the solids atithe indicated level. The fiuidizing gas serves also to strip the vapor-s and gases from the'coke which flows 'down throughthe vessel from pipe 9=as will be later related.

A reduced crude oil to be converted is preferably -preheated to a temperature not above its cracking :temperature, e. g., 700 F. It is introduced into the .bedof hot coke particles via line 2, preferably at a plurality 'of points in the'system. The oil upon contacting :the hot particles undergoes decomposition and the vapors resulting therefrom assist in the fiuidi'zation of theisolids .in the bed and add to its general mobility and turbulent-state. Thepr'oduct vapors pass upwardly through the bed-and are removed from the coking vessel via line 4 after passing through cyclone 16 from-Which solids are returned to the bed via dipleg 7.

A stream of solid .particle's is removed 1from the:coking vessel via line "8 and transported with'the"assistance of air or other free oxygen containing:,gas*'from :line 19 into an external heating zone or burner vessel 10. A transfer line heater could be used 'as an ialternative. Additional air is-supplied -via line 11 to -burner -10. In-the burner a portion of the carbonaceous :materialspre. :g.,

coke or materials deposited thereon, is burned to "raise the temperature to a point sufiicient to supply'the heat to the endothermic reaction occurring-in the coking vessel 1. The temperature of the burner-s'olidsismsually incoming air and resulting combustion gaSes-and' are maintained at a level indicated by the nur'n'eral'lB. "Hot flue'g'ases pass through cyclone '14 and line 16. "'Any en- A higher than minimum re-' trained solids are returned to the bed via dip-pipe 15. A portion of the hot solids are continually removed from burner 10 via line 9 and introduced into vessel 1 at one or more points in order to *maintain heat balance in the system. Fluidizing steam is injected through line 18. Alumina seeds of an average diameter of microns are fed from hopper 20 through line 21 and valve 22 into line 9.

Product coke is withdrawn from burner 10 through line 17 although it can also be taken off from reactor side 1. Because of the relatively small amount of coke burning required for the process the coke particles .containing occluded alumina still retain substantial quantities of the coke material.

In order to express this information more fully the following conditions of operation of the various components are set forth below.

Conditions in fluid coker 1 Green (uncalcined) petroleum coke contains volatile matter which must be removed before the material is suitable for electrode purposes. This removal'is accomplished by calcining the coke at 'high temperature. This is'd'one commercially at temperatures of "1800 F. to 2400' F. or higher'in rotary kilns, vertical retorts, ovens, orthe like. The calcining operation re'duces'the volatile content of coke to 0.5% or lower of'thefinal product, raises'its'true density and'reduces the electrical resistivity of the coke to 0.0015 ohm-inches or lower.

"The calcined coke particles containing the occluded alumina are advantageously produced into carbon "electrodes 'asmentioned before. These are prepared by processing the coke particles-with a 'suitablecoal tar pitch binder asis known'in the art.

In general two types of electrodes are employed by theindustry "(a)' prebaked and (b) 'Soderberg'self-baking electrode. In'theformer, a mixture comprisin'gab'out 78-82% ofcalcined coke aggregate'and l-8'20% of coal tarpitch'is molded at pressures'of about 5000p. s.-i'. or extruded and then "baked for periods up to -30days at 1800-'-2400 F. These preformed electrodes are then use'd'in'ele'ctrolytic cells, beingslowly lowered into the molten alumina'as they are consumed. B-utts'of the uncons'urned electrodes are reground and used in subsequent electrode preparations.

The Soderberg process involves thecontinuous orintermittentaddition of a coke-coal tar pitch-paste'to the top ofthe-cellas the electrode components in *the 'lower'part of the cell areconsumed. *In this operation the paste represents a blend of about 70-72% coke aggregate and 28-30% of pitch. 'The cells'operate attemperatureso'f 950960 'Cxand electrodes are consumed at-therate of about 058 inch per day. 'The paste is baked into 'anelec- "trade by the -l1ot cell gases in the two :months period between the'tirne it'is "added at'thetopa'nd time his used.

'The' net consumption of coke represents 0243 to 0;4'6" lb.

'pefpomfd of' 'a1uminum"metalproduced.

"These electrodes "find greatest utility in their use -as anodesfor the'obtainingof 'aluminumfrom its 'ores by the electrolytic process. The alumina in the anodes continuously dissolves in the bath and thus replenishes a portion of the material consumed during electrolysis.

The arrangement just described is highly flexible. It makes it possible to replace large product coke particles with an equivalent number of small seed particles at so constant and regular a rate as to keep the total number of particles, the particle size distribution, and the total solids inventory substantially constant at all times.

The alumina also serves to lower the sulfur content of the coke produced. The advantage of the presence of bauxite in the carbon electrode as regards the alumina electrolytic process has already been detailed.

Other metal oxides can be employed for seeding the fluid coking process. Thus CaO has utility as a slag former in blast furnaces and MgO is used as a fuel improving agent to depress smoking tendencies.

It is to be understood that this invention is not limited to the specific examples which have been ofiered merely as illustrations and that modifications may be made without departing from the spirit of the invention.

What is claimed is:

1. In a process for coking a heavy hydrocarbon oil by contacting the oil coking charge stock at a coking temperature with a body of coke particles maintained in the form of a dense turbulent fluidized bed in a reaction zone, removing product vapors from the reaction Zone, circulating the coke through an extraneous heating zone wherein a portion of the coke particles are burned, and back to the reaction zone to supply heat thereto, and wherein seed particles are added continuously to the reaction zone in order to counteract excessive coke particle growth, the improvement which comprises utilizing alumina particles as seed to the reaction zone and withdrawing coke particles product containing occluded alumina.

2. The process of claim 1 in which the alumina particles utilized are alumina produced by the purification of bauxite particles.

3. The process of claim 2 in which the alumina particles have a diameter distribution predominantly of to microns.

4. The process of claim 3 in which the amount of alumina utilizedis in the range of 3-10 wt. based on the net coke made in the process.

5. A carbon electrode comprising a baked mixture of a pitch binder and calcined coke particles containing occluded alumina, said coke particles being made by the process of claim 1.

No references cited. 

1. IN A PROCESS FOR COKING A HEAVY HYDROCARBON OIL BY CONTACTING THE OIL COKING CHARGE STOCK AT A COKING TEMPERATURE WITH A BODY OF COKE PARTICLES MAINTAINED IN THE FORM OF A DENSE TURBULENT FLUIDIZED BED IN A REACTION ZONE, REMOVING PRODUCT VAPORS FROM THE REACTION ZONE, CIRCULATING THE COKE THROUGH AN EXTRANEOUS HEATING ZONE WHEREIN A PORTION OF THE COKE PARTICLES ARE BURNED, AND BACK TO THE REACTION ZONE TO SUPLY HEAT THERETO, AND WHEREIN SEED PARTICLES ARE ADDED CONTINUOUSLY TO THE REACTION ZONE IN ORDER TO COUNTERACT EXCESSIVE COKE PARTICLE GROWTH, THE IMPROVEMENT WHICH COMPRISES UTILIZING ALUMINA PARTICLES AS SEED TO THE REACTION ZONE AND WITHDRAWING COKE PARTICLES PRODUCT CONTAINING OCCLUDED ALUMINA. 